1. Field of the Invention
The present invention relates to photographic optical systems suited to silver-halide photographic cameras, video cameras, electronic still cameras or the like and, more particularly, to a diffractive-refractive photographic optical system of large relative aperture having a combination of a refractive optical system and a diffractive optical system and corrected well for the imaging performance of the entire optical system.
2. Description of Related Art
Telephoto lenses have generally a tendency to increase their longitudinal and lateral chromatic aberration as the focal length increases. To correct these chromatic aberrations, it has been the common practice to use a low-dispersion positive lens of fluorite or like material having an extraordinary partial dispersion in combination with a high-dispersion negative lens of another glass material, thereby obtaining achromatism.
However, the extraordinary partial-dispersion glass such as fluorite, though being advantageous for correcting chromatic aberrations, has a drawback that it is very expensive. The specific gravity of the extraordinary partial-dispersion glass is greater than that of the other low-dispersion glasses having no extraordinary partial dispersion. Therefore, the extraordinary partial dispersion glass has another drawback that the whole lens system becomes heavier. For example, fluorite has a specific gravity of 3.18, and FKO1 has a specific gravity of 3.63. On the other hand, FK5, which has small extraordinary partial dispersion, has a specific gravity of 2.46, and BK7, which also has small extraordinary partial dispersion, has a specific gravity of 2.52.
Furthermore, the surface of the extraordinary partial-dispersion glass is relatively susceptible to scratches. Some large-relative-aperture lenses to which the extraordinary partial-dispersion glass is applied are liable to crack when the temperature changes rapidly. Also, in a case where the extraordinary partial-dispersion glass is used in a lens (positive lens) disposed closest to the object side, in order to prevent this lens from being damaged by scratches or cracks, there is a need usually to use a protection glass in the form of a parallel flat plate. So, yet another drawback is produced that the entirety of the lens system increases in weight and cost as much as this protection glass.
With the glasses having no extraordinary partial dispersion left in use, the telephoto lens is corrected for chromatic aberrations by making some other provisions, as in Japanese Laid-Open Patent Application No. Hei 6-324262. In this document, there is disclosed a telephoto lens that includes at least one diffractive optical element having a positive refractive power, at least one refractive optical element having a positive refractive power and at least one refractive optical element having a negative refractive power. That telephoto lens has an F-number of about 2.8 and is relatively corrected well for chromatic aberrations.
However, in that telephoto lens, the diffractive optical element is positioned in a front section of the optical system where both the paraxial on-axial ray and the pupil paraxial ray enter at respective heights (from the optical axis) both of which are relatively large. So, the diffractive optical element gets a large diameter and the production cost thereof becomes high.
For example, to manufacture diffraction gratings at a relatively excellent mass productivity, there is a method of stamping glass by a metallic mold or like means, while melting at a high temperature. Another method is to apply a layer of ultraviolet-ray setting resin to the surface of a glass substrate. A pattern is then formed by a stamping press and then the layer is exposed to ultraviolet rays to harden. Another method is to mold plastic resin itself by a die. In any case, as the diameter of the diffractive optical element increases, the pattern transferability and the die releaseability deteriorate, so that the desired performance or sufficient diffraction efficiency cannot be obtained.
Also, the method of directly cutting glass to form the diffraction grating and the method of wet or dry etching a flat substrate of SiO.sub.2 or others to form the grating grooves in stepwise patterns are usable. However, as the diameter of the diffractive optical element increases, the mass productivity becomes poor and the production cost increases.